1. Field of the Invention
This invention relates to a process that selectively changes the ultraviolet (UV) and visible (VIS) spectral characteristics in silicon nitride and silicon oxynitride thin films.
2. Prior Art
Silicon nitride, chemically represented by Si.sub.x N.sub.y :H.sub.a and silicon oxynitride, chemically represented by Si.sub.x N.sub.y O.sub.z :H.sub.a,OH.sub.b thin films are widely used in the fabrication of very large scale integrated circuits (VLSIC). Often applied as an interlayer dielectric or as the passivating topcoating to provide mechanical protection for the device itself from its environment, they are also used as sacrificial layers in various etch and implant steps. Considerable effort has been devoted to their development as thin gate dielectrics. The application of these films to X-ray and DUV mask technologies is of interest, as well. The SiN:H and SiON:H,OH films can be formed by a variety of conventional deposition technologies, including pyrolitic, plasma enhanced chemical vapor deposition (PECVD), remote plasma enhanced chemical vapor deposition (RPECVD), Infrared-laser induced chemical vapor deposition (LICVD), and Argon Fluoride (ArF) excimer laser assisted CVD (LACVD).
In general, the silicon nitride films are not stoichiometric Si.sub.3 N.sub.4, but rather contain significant amounts of hydrogen present as chemically bound Si--H and N--H. Hydrogen contents ranging from 10 to 30 atomic percent, as quantified by infrared (IR) spectrophotometry, have been reported for PECVD films with the exact amount being a very sensitive function of deposition conditions. Nearly hydrogen-free silicon nitride films have been grown by RPECVD. The situation is similar for silicon oxynitride films, except that the hydrogen can also be incorporated as silanol, Si--OH. Clearly, silicon nitride and silicon oxynitride films must be treated as two classes of compounds with wide ranges in composition. Changes in the Fourier Transform Infrared (FTIR) spectra as the composition is varied indicate that both types must be treated as homogeneous glasses rather than as mixtures of microphases.
The desire to develop silicon nitride as a gate dielectric has focussed much attention on the possible deleterious effects of the bound hydrogen upon the electrical properties of the film. The presence of large quantities of hydrogen has been reported to the detrimental to IC devices. Thermal annealing studies in conjunction with IR spectrophotometry have shown that some of the hydrogen can be removed by heating in a furnace to temperatures in excess of 600.degree. C. Simultaneously, increases in electrical conductivity are observed. However, furnance anneal studies indicate that temperatures exceeding 900.degree. C. induce cracking in the films.
SiN:H and SiNO:H,OH films exhibit a UV absorption band edge that, depending upon composition or processing, can be found at wavelengths as long as 450 nanometers (nm). Early work on silicon nitride films demonstrate a position correlation between a blue shift in the optical absorption band edge and a decrease in the hydrogen content as the films are annealed. Wavelength dispersive x-ray (WDX) spectroscopy is also used to investigate the relationship between the Si/N ratio and the position of the band edge. The existence of an edge in the near-ultraviolet region cannot be explained entirely by the presence of excess silicon in the films. However, the presence of hydrogen as a major component can serve to increase the extent of dislocation and strain within the film, thus contributing to the shift of the band edge towards UV opacity. Studies of silicon oxynitride films indicate that the SiH content exerts the strongest influence on the UV absorption edge. This suggests that the existence of a relatively low lying band edge is due to glass defects associated with SiH sites. While the distortion due to the SiH sites contributes to the alteration of the UV absorbance, SiH sites alone do not completely explain the UV absorbance changes. Defect-defect interactions may also contribute to the changes. Still of the various contributors that determine the UV absorption properties in a deposited film, it is the hydrogen that can most readily be modified to change the UV-VIS characteristics without damaging the film.
Thermal sensitivity and optical absorption in the UV spectral range provide mechanisms for thin film processing by thermal and/or optical means subsequent to deposition. It has been demonstrated that thermal processing can be used to increase UV transparency. However, the required minimum temperature of 600.degree. C., as in the case of furnace anneals, is too high for many applications of interest.
The availability of Argon Fluoride (ArF) excimer lasers offers an attractive alternative to furnace processing. Since the excimer laser operates at 193 nm where these films are generally very absorbing, the heavily hydrogenated nitride or oxynitride films absorb more than 99% of the laser output. This opens the possibility for film-selective thermal annealing in which only the nitride or oxynitride layers reach 600.degree. C. In addition, direct electronic excitation with deep UV (DUV) photons may lead to a weakened Si--H bond so that the dehydrogenation by excimer laser irradiation may be enhanced relative to that of furnace heating. For many applications, the ability to focus the beam to a submicron spot size is a desirable aspect of excimer laser processing. This would allow the user to directly create patterns using the excimer laser processing.
A trend in the semiconductor industry is toward dehydrogenation of the nitride and oxynitride films and striving towards more transparent films and more stable electrical properties. Currently, the whole surface of an erasable programmable read only memory (EPROM) is UV transparent, and erasure by UV is either all or nothing. Using selective dehydrogenation, EPROM chips can be coated with hydrogenated silicon nitride or silicon oxynitride films and areas over individual cells could be made selectively DUV transparent, or left opaque, or placed somewhere in between. Thus, DUV sensitivity, which is currently monitored merely to characterize silicon nitride and silicon oxynitride films becomes another programming element, albeit passive, for devices such as EPROMs. It is also possible to deposit these films on UV transparent substrates, such as quartz, rather than onto VLSI. In this instance, another application would be the advantageous use of heavily hydrogenated blanket films and their characteristic opacity, as bases for DUV masks which can be patterned by directly writing into the films with an excimer laser.